Rate-of-change combustion and combination detection apparatus

ABSTRACT

An ionization-type gas contamination detector, intended primarily for use as a fire detector, in which a substance which emits predominantly low-energy beta particles is used as an ionizing radiation source in an ionizing chamber. Rapid changes in ionization current, which indicate the presence of a fire or smoke preceding a fire, are detected and an alarm is energized by a rate-of-change circuit in response to such changes. The output of the ionization chamber is amplified, and a level detector energizes the alarm when the amplified chamber output reaches a predetermined level. The output of the amplifier is adjusted relatively slowly to a reference level. In one embodiment, the amplifier output is adjusted to the reference level by the activation of a sample-and-hold feedback loop for a short period of time. In another embodiment, continuous feedback through a circuit with a relatively long time constant provides the desired adjustment. This feature makes the device relatively insensitive to normal atmospheric and environmental changes, while being very sensitive to changes in the level of combustion products in the atmosphere.

United Statee tent [191 McMillian et a1.

[ Mar. 19, 1974 RATE-OF-CHANGE COMBUSTION AND COMBINATION DETECTIONAPPARATUS [75] Inventors: Lonnie S. McMillian; George E.

Frohwein, both of Huntsville, Ala.

[73] Assignee: SCI Systems, Inc., Huntsville, Ala.

[22] Filed: Aug. 13, 1970 [21] Appl. No.: 63,646

Related US. Application Data [63] Continuation-impart of Ser. No. 7.444.Feb. 2. 1970.

Primary Examiner-John W. Caldwell Assistant ExaminerDaniel MyerAttorney, Agent, or FirmCurtis, Morris & Safford; Gregor N. Neff 5 7ABSTRACT An ionization-type gas contamination detector, intendedprimarily for use as a fire detector, in which a substance which emitspredominantly low-energy beta particles is used as an ionizing radiationsource in an ionizing chamber. Rapid changes in ionization current,which indicate the presence ofa fire or smoke preceding a fire, aredetected and an alarm is energized by a rate-of-change circuit inresponse to such changes. The output of the ionization chamber isamplified, and a level detector energizes the alarm when the amplifiedchamber output reaches a predetermined level. The output of theamplifier is adjusted relatively slowly to a reference level. In oneembodiment, the amplifier output is adjusted to the reference level bythe activation of a sample-and-hold feedback loop for a short period oftime. In another embodiment, continuous feedback through a circuit witha relatively long time constant provides the desired adjustment. Thisfeature makes the device relatively insensitive to normal atmosphericand environmental changes, while being very sensitive to changes in thelevel of combustion products in the atmosphere.

10 Claims, 10 Drawing Figures LHAMBE Q +12VDC l M6 M4 1 l l 1 A56 1ALARM #64 j 1 524 l I l J PATENTEB MAR 19 I974 SHEET 2 0F 4 W 83 7 fi LWWW o Q Ww my. 1 :Q H 5 fv h /H $11- oh G m ww w 3 m ATTORNEYRATE-OF-CHANGE COMBUSTION AND COMBINATION DETECTION APPTUSCROSS-REFERENCE TO RELATED APPLICATION This patent application is acontinuation-in-part of U. S. application Ser. No. 7,444, filed Feb. 2,1970, now abandoned.

FIELD OF THE INVENTION This invention relates to detection apparatus,and more particularly to fire and gas contamination detection apparatus,and to electronic rate-of-change circuits and ionization chamber devicesused in such apparatus. In an illustrative embodiment described herein,the invention takes the form of an ionizationtype fire alarm system.

OBJECTS OF THE INVENTION It is an object of the present invention toprovide fire and gas contamination detection apparatus which is verysensitive to rapid changes in the condition or composition of theambient gas, but which is relatively sensitive to long-term changes insuch parameters.

Another object of the present invention is to provide ionization-typedetection apparatus having an ionization chamber which does not emit anysubstantial amount of radiation harmful to man.

Yet another object of the present invention is to provide very sensitiverate-of-change electronic detection circuitry suitable for use in theabove-described detection apparatus.

SUMMARY OF THE INVENTION These and other objects are met, in accordancewith the present'invention, by the provision ofa gas contaminationdetector, preferably of the ionization type, preferably for use as acombustion detector, in which the the rate of change of an electricalsignal which varies with the concentration of contaminants is detected.An alarm signal is produced when the rate of change exceeds apre-determined value. In a preferred embodiment, the concentrationsignal is amplified, and timedelayed negative feedback means is used toreduce the output of the amplifier at a pre-determined rate so as tomake the device rate-of-change sensitive. Thus, the device is relativelyinsensitive to normal atmospheric and environmental changes.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be describedwith the assistance of the drawings in which:

FIG. 1 is a schematic diagram of one embodiment of the electronicdetection system of the present invention;

FIG. 2 is a perspective view, shown partly broken away, of ionizationchamber apparatus of the present invention;

FIG. 3 is a detailed schematic circuit diagram of the system shown inFIG. 1;

FIGS. 4 and 5 are graphs showing the variation of certain operatimgparameters of the ionization chamber shown in FIG. 2;

FIGS. 6, 7, 8 and are schematic drawings of further embodiments of theinvention; and

FIG. 9 is a graph illustrating certain operational parameters of theinvention.

DESCRIPTION OF PREFERRED EMBODIMENTS The system shown in FIG. 1 of thedrawings is intended for use primarily as a fire detector and alarmsystem. The entire system includes an ionization chamber 10 whichcontains a radiation source which ionizes the ambient air and producesan ionization current which is inversely proportional to the amount ofheavy gas molecules or other contaminants in the air. A detector circuit40 sets off an alarm 52 when the rate of change in the ionizationcurrent is above a certain level, but does not respond to relatively lowrates of change caused by normal, slow temperature and pressure changesin the air, and other changes in ambient conditions.

As is well known, fires and developing fires in the pre-combustion stageusually cause rapid increases in the concentration of heavy molecules inthe air, and corresponding decreases in ionization chamber currents. Thesystem shown in FIG. 2 detects such changes and activates the alarm 52to warn that a fire has started or is about to start.

TI-IE IONIZATION CHAMBER Referring next to FIG. 2, the ionizationchamber 10 of the present invention includes a tubular housing 14 (shownpartly broken-away). The housing 14 is made of metal, except for aninsulating plate 12 at one end. The housing has a plurality of slots oropenings 16 to permit the gas under test to readily enter the housing.

Two radioactive sources 18 are attached to the metal end wall of housing14. An electronic circuit board 20 (shown partly broken-away) isattached to the insulating base 12 by a plurality of insulating posts22. A metal collector disc 24 is fastened to the circuit board 20 bymeans of a conducting member 26, which serves to rigidly support thecollector disc 24 and to electrically connect disc 24 to the detectorcircuit 40 which is formed on the circuit board 20.

The metal housing 14 and the collector disc 24 form the electrodes ofthe ionization chamber 10. Radioactive sources 18 emit beta particlesinto the gas under test in the housing 14. The beta particles emitted bythe radioactive sources 18 ionize the gas through which they pass. If avoltage is applied between the electrode 14 and the collector disc 24, asmall ionization current will flow between the electrodes 14 and 24 dueto the presence of ions within tubular member 14.

Radioactive sources 18 emit predominantly lowenergy beta particles,which are less harmful to humans than other types of radioactiveemission. A preferred material for the sources 18 is carbon 14 (C nickel63 (N1 tritium (H or technetrium 99 (Tc each of which emits betaparticles almost exclusively and thus makes the ionization chamber ofthe present invention substantially safer than chambers employingsources rich in alpha particles and gamma rays.

Each of the sources 18 preferably is a segment of cylindrical wire. Thewire segments are parallel to one another and are spaced from oneanother so as to provide a relatively uniform flux of beta particles.

As is shown in FIG. 1, the electrodes 14 and 24 are connected to thedetection circuit 40. The presence of gas contaminants within thetubular member 14, such as heavy molecules produced by combustion, ordust, will interfere with the motion of the ionized particles andelectrons so as to reduce the current between electrode 14 and collectordisc 24.

DETECTOR CIRCUIT Referring to FIG. 1, the operation of the detectorcircuit 40 now will be explained. Electrode 14 is connected to a sourceof positive DC bias voltage, e.g., +30V DC. Collector disc 24 isconnected to a current amplifier 42. Current amplifier 42 is connectedto a negative DC bias voltage, e.g., -l2V DC. It is believed that betaparticles (electrons) interact with air molecules and create positiveand negative ions in the chamber 10. The positive ions flow towards thecollector disc 24, and the negative ions flow towards the electrode 14.Thus, in the conventional notation, current flows to the collector discfrom the electrode 14 and through the current amplifier 32. Changes inthis current result in changes in the voltage of electrode 24.

The voltage at electrode 24 is amplified by a voltage amplifier 44. Agate 46 is used to activate a negative feedback loop for the amplifier44. When the gate 46 is closed, the output of amplifier 44 is connectedto the input of amplifier 42, creating a feed-back loop which operatesto drive the output of the amplifier 44 towards zero, by varying thecurrent through the amplifier 42 so as to make the voltage at electrode24 approach zero.

When the gate 46 is opened, the voltage of electrode 24 is initiallymaintained at its previous value by a capacitor 48, which stores andmaintains the previous feed-back signal for the current amplifier 42.The feedback loop now is disabled so that any change in the currentthrough the ionization chamber will result in a substantial change inthe output voltage of the amplifier 44.

A latching circuit 50 is connected in series with an alarm 52 and isconnected to the output of the amplifier 44. A predetermined positivevoltage level at the output of the amplifier 44 will cause the latchingcircuit 50 to conduct and latch, energizing the alarm 52.

The gate 46 is controlled by a clock circuit 54 so as to periodicallyopen and close the feed-back loop. During the time intervals when thegate 46 is closed, the current through the amplifier 42 is readjusted tocorrect for changes in the chamber conditions, such as temperature andpressure, and for amplifier drift. During the time intervals when thegate is open, referred to as the operating intervals, the circuitoperates as a ratesensitive alarm, since a specific change in ionizationcurrent during a single operating interval is required to sound thealarm 52. The specific rate required to sound alarm 52 is electronicallydetermined and may be changed by varying the triggering level of latch50, the open-loop gain of amplifier 44, or the duration of the operatinginterval of clock 54.

A reset switch 56 is provided to silence the alarm by simultaneouslyde-energizing latch 50 and resetting the clock circuit 54 into thecondition in which the gate 46 is closed, thereby readjusting thecurrent through amplifier 42 to the new operating conditions.

Referring to FIG. 3, the details of electronic circuit 40 will now beexplained.

Current amplifier 42 comprises a p-channel, metalon-silicon, fieldeffect transistor 100 connected in a source-follower configuration, withionization chamber serving as the impedance load. A field effecttransistor 100 is employed because the current through ionizationchamber 10 is extremely small, necessitating the use of a currentamplifier capable of delivering extremely small currents. Furthermore,field effect transistor has a high input impedance to prevent thedischarging of capacitor 48 during the operating intervals when gate 46is open.

Amplifier 44 is a three-stage semiconductor amplifier. The first stage,connected to electrode 14, consists of a field effect transistor 102 insource-follower configuration, with a resistor 104 serving as theimpedance load. A field effect transistor 102 is employed to pro vide anextremely high input impedance to prevent loading of the ionizationchamber. A capacitor 106 is also connected to electrode 14, to reducethe high frequency response of amplifier 44 and to bypass to ground any60 cycle AC signals which might be induced in the ionization chambercircuit.

The output of the source-follower stage is connected to the base of asecond stage transistor 198, which is connected in an emitter-followerconfiguration with a resistor 110 serving as the emitter load resistor.

The output of emitter-follower stage is coupled to an integrated circuitoperational amplifier 112 through a resistor 114. A resistor 116 couplesthe output of operational amplifier 112 with its input, to providenegative feedback and thus limit the gain of operational amplifier 112.Source-follower transistor 102 and emitter follower transistor 108provide current gain for the voltage signal present at electrode 24.Operational amplifier 112 provides voltage gain for this signal, so thatthe voltage gain of amplifier 44 is essentially determined by resistors114 and 116, which determine the gain of operational amplifier 112.

The output lead 117 of the operational amplifier 112 is connected to thelatching circuit 50. The latching cir cuit 50 includes a transistor 118,and the lead 117 is connected to the base lead of transistor 118 througha resistor 120 and a diode 122. Transistor 118 is connected in acommon-emitter configuration, with a resistor 124 and alarm 52 providingthe collector load. Transistor 118 normally is non-conducting. Theappearance of a positive voltage above a predetermined level on theoutput lead 117 of the operational amplifier 112 will provide base drivecurrent for transistor 118, and will cause it to conduct. The level ofvoltage required is determined by the value of resistor 120. Diode 122prevents transistor 118 from being damaged by excessive negative biasshould the output of operational amplifier 112 be negative. A capacitor126 is connected to the base of transistor 118 to reduce the highfrequency response of latching circuit 50 to avoid false alarms whichotherwise might be caused by transient voltages at the output ofoperational amplifier 112.

The collector lead of transistor 118 is connected to the base of anothertransistor 128, which is connected in common emitter configuration, witha resistor 130 serving as a collector load resistor. Transistor 128 alsonormally is non-conducting. When transistor 188 conducts, the voltage atthe base of transistor 128 will be substantially reduced, causingtransistor 128 to conduct. Alarm 52, which is in series with transistors118 and 128, will thus be energized when transistors 118 and 128 arecaused to conduct by the presence of the predetermined positive voltagelevel at the output of operational amplifier 112.

The latching function of latching circuit 50 is provided by theconnection of the collector of transistor 128 to the base of transistor118 so as to provide positive feed-back to transistor 1 18. Thispositive feedback signal will cause transistor 118, and thus transistor128, to remain conducting despite any change in the output voltage ofoperational amplifier 112. Thus, due to the positive feed-back signal,alarm 52 will continue to be energized despite any change in the outputvoltage of operational amplifier 112.

Gate circuit 46 comprises a p-channel, metal-onsilicon field effecttransistor 132, employed as a switch. The output signal from operationalamplifier 112 is conducted to transistor 132 through a resistor 134which is connected to the source of transistor 132. The drain oftransistor 132 is connected to the input of current amplifier 42. Thesubstrate of transistor 132 is connected to the +12 volt DC supply, sothat a negative voltage applied to the gate of transistor 132 will causeit to conduct, thereby conducting the output signal of operationalamplifier 1 12 to the input of current amplifier 42 and to the capacitor48.

The signal for the gate of transistor 132 is generated by the clockcircuit 54, whose operation will be described in sequence, commencingwith the start of the operating interval. A capacitor 136 is connectedin series with one of two resistors 138 and 140, depending upon theposition of a sensitivity selection switch 142. At the start of theoperating interval, capacitor 136 will be substantially discharged.Thereafter, it will charge at a rate determined by the RC time constantof resistor 138 or 140 and capacitor 136, depending upon which positionswitch 143 is in. Capacitor 136 charges until it reaches a voltage levelsufficient to cause a junction field effect transistor 142 to conduct.The gate of transistor 142 is connected to the junction of capacitor 136and resistors 138 and 140. The charging time of capacitor 136 determinesthe operating time interval of the detection circuit 40, and the lengthof that time interval is a factor in the determination of thesensitivity of the system. A longer time interval, corresponding to theuse of the larger of the resistors 138 and 140, produces greatersensitivity, since the required change in ionization current may occurover a longer time interval. The rate of change detection sensitivity ofthe circuit 40 is, therefore, varied by operation of switch 143. Thepreferred operating time intervals employed in typical fire alarmsystems using the invention vary from seconds to several minutes.

The base of a transistor 144 is connected to the load resistors 146 and148 of transistor 142. Transistor 144 is normally conducting, so that itwill assume a nonconducting state when transistor 142 conducts. The baseof transistor 150 is connected to the collector load resistors 152 and154 of transistor 144. Transistor 150 is normally non-conducting, sothat it will conduct when transistor 144 assumes a non-conducting state.A resistor 156 serves as a collector load resistor for transistor 150.The collector of transistor 150 is connected to the gate of transistor132 in the gating circuit 46, so that transistor 132 will conduct whentransistor 150 conducts.

The base of a transistor 158, connected in a commen-emitterconfiguration, with a resistor 160 serving as a collector load, isconnected to transistors 144 and 150 through a RC network comprising apair of resistors 162 and 164 and a capacitor 166. Transistor 158 isnormally non-conducting. After a time interval, during which capacitor166 is charging, transistor 158 will conduct. This causes capacitor 136to discharge through transistor 158, and thus resets transistors 142,144, and 150 to their initial condition. Transistor 150 is once againnon-conductive, thereby causing transistor 132 to assume anon-conducting state. This second time interval during which transistor132 is conductive, corresponds to the time interval during which thenegative feedback loop for amplifier 44 is closed. In a typical firealarm system, this time interval is substantially shorter than theoperating time interval, and usually is several seconds long.

Reset switch 56 serves a dual function. When actuated, reset switch 56disconnects the +1 2V supply from the alarm 52 and latching circuit 50,thereby silencing the alarm and disabling the latching circuit. Inaddition, gate drive current to transistor 142 is supplied throughresistor 16Q, causing tansistor 142 to conduct, thereby placing theclock circuit in the condition in which transistor 132 is conductive.This closes the negative feedback loop for amplifier 44, so that theoutput of operational amplifier 112 will once againg be driven to zero.This insures that the input to the latching circuit 50 will be zero andthat the alarm will remain in an off condition immediately after releaseof the reset switch 56.

IONIZATION CHAMBER OPERATING PARAMETERS FIGS. 4 and 5 show thevoltage-current characteristics of an ionization chamber 10 which hasbeen built and successfully tested. This chamber had the followingdimensions:

Housing 14 diameter 3 inches Collector disc 24 diameter 2 inches Spacingfrom collector disc 24 to radioactive sources 18 4 cm.

Radioactive source 18 microcuries Ni FIG. 4 is a graph showing thevoltage-current characteristics of the chamber 10 at two differentambient gas pressures, P and P Pressure P is approximately equal toatmospheric pressure at sea level, and P is approximately equal toatmospheric pressure at an altitude of 10,000 feet. Applicants haverecognized that operation of the chamber 10 at bias voltages below theknees in the curves P and P i.e., at voltages less than about 20 volts,will make the ionization current unduly sensitive to ambient gaspressure changes. Applicants also have recognized that the spacingbetween the curves P and P increases as the voltage is increased muchabove about 40 volts, with the result that the sensitivity to pressurechanges will increase with increasing chamber voltages above the level.Thus, the ideal bias voltage range is within the saturation region ofthe chamber, identified by the nearly horizontal portions of the curvesP and P but not far into the saturation region. In the specific exampledepicted in FIGS. 4 and 5, the preferred range is 20 to 40 volts, andthe preferred voltage is 30 volts. The preferred region of operation isreferred to herein as the low-voltage saturation region.

FIG. 5 is a graph similar to that of F IG. 4, except that the gaspressure remains constant while the gas temperature is changed from T toT T is approximately 10 centigrade and T is approximately 60 centigrade.Once again, the best operating region proves to be the low-voltagesaturation region, since voltages above or below this region producegreater sensitivity to gas temperature changes. A voltage of 30 volts isbest for the specific configuration whose operation is depicted in FIG.5.

CONTINUOUS FEEDBACK EMBODIMENTS The combustion detector 200 shown inFIG. 6 includes an ionization chamber 202 having a beta particlecollector housing 204, a radiation source 206 which is attached to ananode plate 208. In addition, a guard ring" 210 is provided. The guardring 210 surrounds the anode plate 208 and is maintained atsubstantially the same voltage as the anode plate in order to interceptand collect stray currents which otherwise might flow to the anodeplate. Ambient air can enter and leave the chamber through the gapbetween the plate 208 and the housing 204.

The radiation source 206, like the source 18, produces predominantlybeta particles and preferably consists of carbon 14 (C The collectorhousing 204 of chamber 202 is connected to a line 220 which is at zeropotential. The anode plate 208 of the chamber is supplied with anegative bias voltage which appears on a lead 218. Thus, a voltage isapplied across the ionization chamber.

The lead 218 is connected to the gate electrode of a first field effecttransistor (FET) 214 of a pair 212 of matched junction FETs 214 and 216.As in the previous embodiments, the use of a field effect transistor asthe first stage in the amplification chain presents a very high inputimpedence to the anode 208. The signal from each of the FETs isconducted to one of two input leads of a high-gain operationaldifferential amplifier 222. By using matched FETs, the FET gatesourcevoltage drop components of the voltages at the two inputs to theamplifier 222 will be approximately equal and will be balanced-out bythe differential operation of the amplifier.

The output signal from the amplifier 222 is conducted to an integratingcircuit 223 consisting of a resistor 234, a capacitor 236, a resistor234 and a transistor 238. The output of this integrating circuit appearsacross the load resistor 248 and is fed back to the input of the fieldeffect transistor pair 212 through a resistive network includingresistors 240, 242, 244 and 246. The amplifier 222 amplifies the signalit receives from the ionization chamber 202. This signal is integratedby the integrating circuit and the signal from the integrator is fedback to change the bias signal on FET pair 212, thus changing the inputbias for the amplifier 222 and reducing its output signal. The output ofthe amplifier 222 is conducted to an alarm circuit (to be described)which produces an alarm signal when the level of the output of amplifier222 reaches a predetermined value.

The values of the resistors 234 and 237, the capacitor 236, and theother components in the feedback network are chosen so as to give thenetwork a relatively long time constant. For example, a time constant of20 minutes has been found to be desirable and practical. Thus, theintegrator circuit provides a negative feedback signal which increasesrelatively slowly with time. The reason for this is to make the devicesensitive to combustion products which accumulate relatively rapidly,thus indicating a fire or incipient fire, and yet prevent the devicefrom alarming in response to relatively slowly changing atmosphericconditions. For example, with a time constant of twenty minutes orthereabouts, the device has alarmed in response to an accumulation ofsmoke emitted by smoldering wood, but does not alarm when atmosphericpressure or temperature, etc. change at an ordinary rate. After arelatively long time, the negative feedback circuit will return the biassignal on the FET pair 212 to approximately its original value. Thus,relatively slow changes in ionization current will be cancelled out bythe system.

Referring now to the right-hand portion of F IG. 6, an alarm device 274is energized by means of a transistor 260 which energizes asilicon-controlled rectifier (SCR) 266 which, in turn, energized a relay270 to close a switch 272 and turn on the alarm device. The base lead ofthe transistor 260 is connected through a resistor 250 to the output ofamplifier 222. A capacitor 256 is connected between the base lead andthe emitter lead oftransistor 260. The RC circuit formed by the resistor250 and the capacitor 256 delays the build-up of input signal to thetransistor 260 for a relatively short length of time in order to insurethat the signal being received is not caused by transients such as thosewhich might occur due to puffs of air being blown into the ionizationchamber 202.

When the output of the amplifier 222 reaches a predetermined level,which is determined by a pair of resistors 252 and 254 connected in avoltage divider arrangement, and after the slight time delay mentionedabove, transistor 260 turns on and conducts current through a loadresistor 262. The load current flows through another resistor 264 with acapacitor 258 connected in parallel with it and to the gate lead of theSCR 266 to turn the SCR on. This causes current to flow through the SCR,through a voltage dropping resistor 268 and the coil of relay 270 toclose the contact 272 and energize the alarm 274. Also, a lamp 276 islighted to indicate that the device 200 is in an alarm condition. Thealarm device 274 itself may be situated at a remote location,particularly if the fire detection unit 200 is merely one of anextensive fire detection system. The SCR 266 will remain latched in theon" condition until the voltage applied to it is removed forapproximately 15 seconds. This can be done by a switch (not shown) whoseoperation will reset the alarm device 274.

The integrator device 223 is commonly known as a Miller" integrator. Thetransistor 238 is used for amplification and inverting functions, aswell as its functions in the integrator circuit. The resistor 237stabilizes the circuit and reduces the effect of transients.

The resistors 240 and 242 comprise a voltage divider network whichreduces the gain in the feed-back loop and reduces the rate at which thefeed-back voltage increases. The resistance of resistor 240 is 10 timesthat of resistor 242; resistor 240 has a preferred resistance of 2megohms, and resistor 242 has a resistance of 200,000 ohms.

Resistor 244 is a relatively large (e.g., 4.7 megohms) resistor which isused to adjust the bias level of the chamber housing 204. Resistor 246is a very large resistor (e.g., 1,000 megohms) which converts ionizationcurrent changes into voltages, without unduly loading the chamber.Capacitor 245 is connected across resistor 246 in order to filter outstray 60 Hz and highfrequency noise signals which might be picked up inthe circuit. Another 60 Hz and noise filter capacitor 232 is connectedbetween the output of the amplifier 222 and a line leading to the gatelead of PET 216. A variable resistor 230 is connected between the sametwo points in order to adjust the gain of the amplification provided bythe FET pair 212 and the amplifier 222.

The guard ring 210 is connected to the gate lead of FET 216 through avery large (e.g., 1,000 megohms) resistor 226. Thus, because of the highinput gate impedances of the FETs, the guard ring 210 is maintained atsubstantially the same potential as the anode plate 208.

POWER SUPPLY A 24 volt direct current supply signal is applied to inputterminal 300 and 302. A steady, regulated voltage of-lO volts ismaintained on line 229 by the connection of a Zener diode 282 and afilter capacitor 278 between the input lead 300 and the point 279.Similarly, a regulated voltage of volts is maintained between points 279and 281 by means of a second Zener diode 284 and filter capacitor 280.

Two trouble contacts 296 are provided. The appearance of a closedcircuit at those terminals indicates that line voltage is being suppliedto the fire alarm system, and that Zener diodes 280 and 284 have notbeen short circuited. The relay is maintained in a energized conditionby current flowing to its coil through a transistor 290, and this keepsthe contact 295 of the relay closed. Transistor 290 receives biasenergization by means of a voltage divider consisting of two resistors286 and 288 whose midpoint is connected to the base lead of transistor290, and the combination is connected between the negative 24 volt lineand point 281. When the line opens or one of the Zener diodes 282 or 284becomes short-circuited, the relay 294 will be deenergized and troublewill be indicated by the open circuit between trouble terminals 296. Adiode 298 is connected between the emitter of transistor 290 and theinput lead 302.

Following is a table illustrating the specific circuit components whichwere used in a device which was successfully built and tested inaccordance with the embodiment of the invention shown in FIG. 6:

Part No. Value or identification FET pair 212 2 N 3958 Amplifier 222Operational differential amplifier SN 7274l sold by Texas Instruments,Corp. 2 N 2907 4.7 megohms 470,000 ohms.

640 microfarads Transistors 238 and 260 Transistor 290 Resistor 234Resistor 237 Capacitor 236 Resistor 248 l 10,000 ohms. Resistor 240 2megohms Resistor 242 200,000 ohms. Resistor 244 4.7 megohms Resistor 246I000 megohms Capacitor 245 Resistor 226 500 Picofarads I000 megohms Eachof the 1,000 megohm resistors preferably consists of a rod of ceramicmaterial with a metal oxide and ceramic coating. Such resistors are soldby Welwyn, Inc. The capacitor 236 preferably is a low-leakage capacitorwith polycarbonate film dielectric material, sold by Comell-Dublier.

The preferred embodiment of the present invention is shown in FIG. 7 ofthe drawings. The circuits shown in FIG. 7 is, in substance, the same asthat shown in FIG. 6, except that the circuit is considerably simplifiedso as to reduce the number of components. The same reference numeralsare used for the same components in both FIGS. 6 and 7. The mostimportant changes are in the feed-back network from the amplifier 222,and only those changes will be described in detail.

A 100,000 ohm resistor 354 is connected to the out put lead of theamplifier 222 at a terminal 350. A 5 microfarad polycarbonate,low-leakage capacitor 356 is connected to the opposite terminal ofresistor 354, as is a ohm resistor 352. The outer terminal of resistor352 is connected through the low-impedance path of the coil of relay 294to the negative terminal of the power supply. A 5,000 megohm ceramicresistor 358 is connected between the terminal 350 and the other end ofthe capacitor 356. The common terminal 357 between resistor 358 andcapacitor 356 is connected to a parallel connection of 500 picofaradcapacitor 360 and another 5,000 megohm ceramic resistor 362. Thatparallel combination is connected to the plate 208 of the ionizationchamber 202 and the gate lead of FET 216, to complete the feed-backpath. A Zener diode 264 is connected between the point 350 and the inputof the SCR 266 to set a bias level for the alarm voltage.

FIG. 8 is a simplified schematic circuit diagram for the feed-backamplifier and ionization chamber connections of the circuit shown inFIG. 7. It is believed that the following equation expresses therelationship between a change in input voltage E and a change in outputvoltage E, at the output terminal 350:

In which:

AE the change in output voltage at terminal 350;

AE the change in input voltage;

R and R are the resistors shown in FIG. 8.

In the above equation, an approximation has been made in view of thefollowing relationships:

b. (-K), the open loop gain of amplifier 222, is

c. E the change in chamber current, 1

divided by R,,.

d. The change in input current to the amplifier The above equation isbelieved to hold true for ionization current changes which occur over atime period that is short relative to the time constant of resistor 358and capacitor 356 and long relative to the time constant of resistor 362and capacitor 360. FIG. 9 shows the relationship which is believed toexist between the voltage gain of the circuit and the frequency of theinput signals. In FIG. 9, the following equations hold true:

f1 a C1) f2 4 2) in which C and C are the capacitors shown in FIG. 8,

and f and f are the frequencies at which the gain has dropped 3 decibelsfrom the maximum. These frequencies indicate the pass band of thecircuit.

As is shown by FlG. 9, the circuit gives little or no amplification forrelatively high or very low frequency input signals. This prevents thedetector from alarming in response either to electrical noise or stray60 H, signals (both having relatively high frequency components) or toslowly changing input signals caused by normal atmospheric changes.

The gain of the circuit preferably is 1.0 to DC. input signals. However,withou amplification, signals from the ionization chamber are not largeenough to enable the alarm. Slowly changing input signals, such as thosecaused by changing atmospheric conditions, cause slow increases inoutput voltage from the amplifier 222. However, those changes are soslow that the alarm level of the amplifier is not reached because thenegative feed-back voltage of the feed-back network increases fastenough to prevent the amplifier output from reaching the alarm level.

Input signals having relatively high-frequency components, such as noisespikes and 60H signals, flow through the capacitors 360 and 356 and the100 ohm resistor 352 to ground rather than to the high impedance (loohms) gate lead of the FET 16. Thus, such high-frequency components arefiltered out of the amplifier input.

The frequencies f, and f are selected such that input signals having thefrequency components caused by the usual fire will receive fullamplification, and signals not having those components will receivereduced amplification. Since the frequency of the components of a signalis a function of the rate of change of the signal, the alarm device isresponsive to the rate of change of the combustion products signal.

The alarm device has a dead-band" in that regardless of the rate ofchange of the input signal, unless the magnitude of the output signalfrom the amplifier reaches a certain minimum value, the device alarmremains disabled. This minimizes the flase-alarming due to smallcombustion or contamination sources, such as ciagarette smoke, etc.

The detector system described above has a number of advantages. It is avery sensitive detector of fires and gas contaminants, and yet it isrelatively insensitive to normal ambient temperature and pressurechanges. Moreover, these desirable features are obtained with only oneionization chamber, and with the use of reliable solid-state circuitry.

The detector also is relatively small, and thus easily can be madeinconspicuous in factories, offices and other structures in which itmight be used. The detector also is relatively light in weight, afeature which makes its use highly advantageous in many uses.

The detector utilizes a radiation source, such as carbon 14 or nickel63, which emits predominantly beta particles, and therefore emitspractically no harmful radiation. This makes the device inherently safefor use in human dwellings and permits a reduction in safety maintenancecosts. Furthermore, such materials are relatively inexpensive to use asradiation sources.

FIG. shows another embodiment of the invention. This embodiment is aso-called smoke detector, i.e., a detector in which a light source 284emits light through a lens system 286 in a beam 388. The beam 388 oflight is received by an alarm unit 390 which has a lens system 392 and aphotoconductor 294 which receives light from the lens system 392.

When smoke or other light-blocking contamination intercepts the lightbeam 388, the resistance of the photoconductor 394 increases. A knownalarm circuit is connected to the photoconductor 394 and alarms inresponse to the change of resistance, thus indicating the presence ofsmoke. The smoke detector described so far is conventional.

The lenses of such a system often become coated with dust or otherpollutants. This adversely affects the operation of the smoke detector.In accordance with the present invention, the alarm circuit 396 of thepresent invention is used in place of the conventional alarm circuit.The circuit is connected as shown in FIG. 7, for example, except thatthe photoconductor 394 replaces the ionization chamber. The resultingdetector has the advantage that the usual coating of the lenses iseliminated as a source of alarm signals since the signal changes createdby such coating increase very slowly. The effects of such changes areeliminated in the same way that changes due to atmospheric changes areeliminated. Thus, the detector is more reliable and requires lessmaintenance.

The above description of the invention is intended to be illustrativeand not limiting. Various changes or modifications in the embodimentsdescribed may occur to those skilled in the art and these can be madewithout deaprting from the spirit or scope of the invention.

We claim:

1. A contamination detector comprising means for developing anelectrical signal which varies with the concentration of gas moleculesin an ambient medium, and alarm means for producing an alarm signal whenthe rate of change of said electrical signal exceeds a pre-determinedvalue, said signal developing means including amplifier means foramplifying said electrical signal, and said alarm means including meansfor reducing the output signal from said amplifier means at apre-determined timedelayed rate.

2. A detector as in claim 1 in which said alarm means includes means forpreventing the production of an alarm signal in response tohigh-frequency components of said electrical signal.

3. A detector as in claim 1 in which said signal developing meansincludes a light transmitter emitting a beam of light, a light receiverreceiving and directing said beam to a photosensitive circuit element.

4. A combustion detector comprising means for developing an electricalsignal which varies with the concentration of gaseous molecules ofcombustion products in an ambient medium, and alarm means for producingan alarm signal when the rate of change of said electrical signalexceeds a pre-determined value, said signal developing means includingamplifier means for amplifying said electrical signal, and said alarmmeans including means for reducing the output signal from said amplifiermeans at a pre-determined time-delayed rate.

5. A detector as in claim 4 in which said alarm means includes means fordisabling said alarm means until said electrical signal reaches apre-determined minimum vlaue, and for preventing the production of analarm signal by said alarm means in response to highfrequency componentsof said electrical signal.

6. A combustion detector comprising, in combination, an ionizationchamber, means for conducting electrical current through said chamber,means for producing a detector signal proportional to said current,means for amplifying said detector signal, time delay negative feedbackmeans for feeding back from the output to the input of saidamplification means a feedback signal which is a time-delayed functionof the outputput signal of said amplification means, and alarm means foralarming when the output of said amplification means reaches apredetermined alarm level.

7. A detector as in claim 6 in which said feedback means includes anintegrator.

8. A detector as in claim 6 including time delay means coupling theoutput of said amplification means to said alarm means for delaying thechange in said output and preventing false-alarming due to transients.

9. A contamination detector comprising means for developing anelectrical signal which varies with the concentration of contaminants inan ambient medium, said signal developing means including means foramplifying said electrical signal, and alarm means for producing analarm signal when the rate of change of said electrical signal exceeds apre-determined value, said alarm signal producing means including asignal level detector means for detecting the output signal level ofsaid amplifier means, and negative feedback means for feeding back tothe input of said amplifier means a signal which is a time-delayedfunction of the output signal of said amplifier means.

10. A detector as in claim 9 in which said feed-back signal isproportional to the time integral of said amplifier output signal.

1. A contamination detector comprising means for developing anelectrical signal which varies with the concentration of gas moleculesin an ambient medium, and alarm means for producing an alarm signal whenthe rate of change of said electrical signal exceeds a pre-determinedvalue, said signal developing means including amplifier means foramplifying said electrical signal, and said alarm means including meansfor reducing the output signal from said amplifier means at apre-determined time-delayed rate.
 2. A detector as in claim 1 in whichsaid alarm means includes means for preventing the production of analarm signal in response to high-frequency components of said electricalsignal.
 3. A detector as in claim 1 in which said signal developingmeans includes a light transmitter emitting a beam of light, a lightreceiver receiving and directing said beam to a photosensitive circuitelement.
 4. A combustion detector comprising means for developing anelectrical signal which varies with the concentration of gaseousmolecules of combustion products in an ambient medium, and alarm meansfor producing an alarm signal when the rate of change of said electricalsignal exceeds a pre-determined value, said signal developing meansincluding amplifier means for amplifying said electrical signal, andsaid alarm means including means for reducing the output signal fromsaid amplifier means at a pre-determined time-delayed rate.
 5. Adetector as in claim 4 in which said alarm means includes means fordisabling said alarm means until said electrical signal reaches apre-determined minimum vlaue, and for preventing the production of analarm signal by said alarm means in response to high-frequencycomponents of said electrical signal.
 6. A combustion detectorcomprising, in combination, an ionization chamber, means for conductingelectrical current through said chamber, means for producing a detectorsignal proportional to said current, means for amplifying said detectorsignal, time delay negative feedback means for feeding back from theoutput to the input of said amplification means a feedback signal whichis a time-delayed function of the outputput signal of said amplificationmeans, and alarm means for alarming when the output of saidamplification means reaches a predetermined alarm level.
 7. A detectoras in claim 6 in which said feedback means includes an integrator.
 8. Adetector as in claim 6 including time delay means coupling the output ofsaid amplification means to said alarm means for delaying the change insaid output and preventing false-alarming due to transients.
 9. AconTamination detector comprising means for developing an electricalsignal which varies with the concentration of contaminants in an ambientmedium, said signal developing means including means for amplifying saidelectrical signal, and alarm means for producing an alarm signal whenthe rate of change of said electrical signal exceeds a pre-determinedvalue, said alarm signal producing means including a signal leveldetector means for detecting the output signal level of said amplifiermeans, and negative feedback means for feeding back to the input of saidamplifier means a signal which is a time-delayed function of the outputsignal of said amplifier means.
 10. A detector as in claim 9 in whichsaid feed-back signal is proportional to the time integral of saidamplifier output signal.